Quantitative characterization of scratch and mar behavior of polymer coatings

Scratch and mar damage of automotive topcoats is of increasing concern to the automobile industry and its customers. To better characterize and understand the scratch and mar behavior, we developed a nanoscratch technique to measure important mechanical quantities with high precision. With this technique, mar behavior can be studied in two steps: the damage formation process and the relationship between damage morphology and visual impact. The scratch and mar performance can be evaluated based on plastic and fracture resistance of coatings. In this paper, we will focus on micro-mechanical aspects of scratch deformation. A detailed investigation suggests that the oscillation of measured quantities during scratch damage results from fracture not stick-slip events. For the case discussed here, cracks start in the scratch direction and then gradually propagate 30° away from the direction, which suggests cracks were initiated by tensile stress in front of the indentor.